The United States would welcome a move by Japan to extend air patrols into the South China Sea as a counterweight to a growing fleet of Chinese vessels pushing Beijing's territorial claims in the region, a senior U.S. Navy officer told Reuters.

Currently, regular patrols by Japanese aircraft only reach into the East China Sea, where Tokyo is at loggerheads with Beijing over disputed islands. Extending surveillance flights into the South China Sea will almost certainly increase tensions between the world's second- and third-largest economies.

"I think allies, partners and friends in the region will look to the Japanese more and more as a stabilizing function," Admiral Robert Thomas, commander of the Seventh Fleet and the top U.S. navy officer in Asia, said in an interview.

"In the South China Sea, frankly, the Chinese fishing fleet, the Chinese coast guard and the (navy) overmatch their neighbors," Thomas said.

China’s foreign ministry said it had no immediate comment on the interview.

Thomas's comments show Pentagon support for a key element of Japan Prime Minister Shinzo Abe's push for a more active military role in the region. That is crucial because U.S and Japanese officials are now negotiating new bilateral security guidelines expected to give Japan a bigger role in the alliance, 70 years after the end of World War Two.

A Chinese coast guard ship sprays water cannon on a Vietnamese coast guard to protect a giant oil rig China deployed illegally in Vietnam's exclusive economic zone in June 2014.

Japan is not party to the dispute in the South China Sea where China, the Philippines, Vietnam, Taiwan, Malaysia and Brunei have competing claims. But the waterway provides 10 per cent of the global fisheries catch and carries $5 trillion in ship-borne trade, a large portion of which is to and from Japan.

New surveillance plane

Abe is pushing for legislation later this year that would allow Japan's military to operate more freely overseas as part of a broader interpretation of the self-defense allowed by its pacifist constitution.

Those changes coincide with the deployment of a new Japanese maritime patrol plane, the P-1, with a range of 8,000 km (5,000 miles). That is double the range of current aircraft and could allow Japan to push surveillance deep into the South China Sea.

"This is a logical outgrowth of Abe's push for a more robust and proactive military. It is also a substantial departure from JSDF's customary operations," said Grant Newsham, a research fellow at the Japan Forum for Strategic Studies and a former U.S. Marine liaison officer to Japan's military.

Newsham said sending surveillance aircraft to the South China Sea would allow Japan to deepen its military ties with nations like the Philippines, one of Abe's goals to counter China's growing naval power.

Beijing has outlines the scope of its claims with reference to a so-called nine-dash line that takes in about 90 percent of the South China Sea on Chinese maps.

"The alleged nine dash line, which doesn’t comport with international rules and norms, standards, laws, creates a situation down there, which is unnecessary friction," said Thomas, the U.S. navy commander.

The Scarborough Shoal near the Philippines is one flashpoint in the South China Sea. Manila has complained that China has kept its fishermen from fishing in the waters around the shoal. Thomas said Japan could aid the Philippines with equipment and training.

"For the Philippines, the issue is one of capacity. For the Japanese that is a perfect niche for them to help, not just in equipment, but in training and operations," the U.S. Seventh Fleet commander said.

Centered around the USS George Washington carrier battle group with its home port in Japan, the U.S. Seventh Fleet includes some 80 vessels, 140 aircraft and 40,000 sailors making it the most powerful naval force in the western Pacific.